The present invention relates to a received signal strength detecting circuit and, more particularly, to a received signal strength detecting circuit which permits response time reduction.
A received signal strength detecting circuit (or an RSSI circuit) is used in a receiver for accurately detecting-the received signal strength (or reception level).
FIG. 6 is a block diagram showing a prior art received signal strength detecting circuit. A received input signal 100 is converted in a reception circuit 200 into a received IF signal 110 and then demodulated in a demodulator 210 to obtain demodulated data 120.
The reception circuit 200 also outputs a reception level signal 160 representing the amplitude level of the received input signal 100. This reception level signal 160 is usually outputted from an IF amplifier in the reception circuit 200.
In an FM modulation system, the received level signal 160 has a constant value. In a modulation system in which the amplitude is variable, such as an AM or a PSK modulation system, however, the signal 160 has a variable amplitude level component in dependence on the modulation content. In order to obtain an RSSI signal 180 which is proportional to the level of the received input signal 100 regardless of the modulation content, therefore, the reception level signal 160 should be smoothed through an LPF 220. As a result of this smoothing, the variable amplitude level component which is dependent on the modulation content, is averaged, and a value independent of the modulation content can be obtained. This type of receiver is described in, for instance, Japanese Laid-Open Patent Publication No. 4-56529.
The LPF 220 is an inevitable element for suppressing variations of the reception level due to amplitude level variations. Japanese Laid-Open Patent Publication No. 3-291027, for instance, discloses a technique of switching the time constant of the LPF to meet requirements in mobile communication systems.
However, as a result the passing of the reception level signal 160 through the LPF 220, the output RSSI signal 180 has a dull waveform. In the prior art, therefore, quick amplitude level variations of the received input signal 100 can not be followed up.
In order to be able to follow up the level variations of the received input signal 100, it is conceived to reduce the extent of dulling of the waveform by reducing the time constant of the LPF 220. W the extent of duling of the waveform is reduced, however, the variable amplitude level component due to the modulation is no longer ignorable, resulting in such an error as the introduction of the variable amplitude level component in the RSSI signal 180.
In addition, the LPF 220 slows down the response of the received level signal, in the prior art the rise time of the reception circuit from the instant of the power source connection can not be reduced. In order for the reception level to be sufficiently converged in a necessary timing when the receiving circuit is turned on, the power source for the reception circuit should be connected at an earlier timing.
FIG. 7 shows waveforms involved when the power source is connected for intermittent signal reception. In a system in which signal is received intermittently such as waiting reception frequently used in recent mobile communication, the use of the above long rise time reception circuit results in a large "on" time ratio to the full time, and makes it impossible to reduce the average current consumption.